Voicing occurs when air is expelled from the lungs through the glottis, creating a pressure drop across the larynx. When this drop becomes sufficiently large, the vocal folds start to oscillate. The minimum pressure drop required to achieve phonation is called the phonation threshold pressure, and for humans with normal vocal folds, it is approximately 2-3 cm H2O. The motion of the vocal folds during oscillation is mostly laterally, though there is also some superior component as well. However, there is almost no motion along the length of the vocal folds. The oscillation of the vocal folds serves to modulate the pressure and flow of the air through the larynx, and this modulated airflow is the main component of the sound of most voiced phones.
The vocal folds will not oscillate if they are not sufficiently close to one another, are not under sufficient tension or under too much tension, or if the pressure drop across the larynx is not sufficiently large. In linguistics, a phone is called voiceless if there is no phonation during its occurrence. In speech, voiceless phones are associated with vocal folds that are elongated, highly tensed, and placed laterally (abducted) when compared to vocal folds during phonation.
Fundamental frequency, the main acoustic cue for the percept pitch, can be varied through a variety of means. Large scale changes are accomplished by increasing the tension in the vocal folds through contraction of the cricothyroid muscle. Smaller changes in tension can be effected by contraction of the thyroarytenoid muscle or changes in the relative position of the thyroid and cricoid cartilages, as may occur when the larynx is lowered or raised, either volitionally or through movement of the tongue to which the larynx is attached via the hyoid bone. In addition to tension changes, fundamental frequency is also affected by the pressure drop across the larynx, which is mostly affected by the pressure in the lungs, and will also vary with the distance between the vocal folds. Variation in fundamental frequency is used linguistically to produce intonation and tone.
The voicing mechanism that is specifically designed for voice production is the larynx. The larynx is between the pharynx and the trachea. It communicates with the mouth and the nose though the laryngeal and oral parts of the pharynx. Although the larynx is part of the air passages, the larynx normally acts as a valve for preventing swallowed food and foreign bodies from entering the lower respiratory passages. The larynx is located in the anterior portion of the neck.
The laryngeal skeleton comprises nine cartilages that are joined by various ligaments and membranes. Three of the cartilages are single (thyroid, cricoid and epiglottis), and three are paired (arytenoid, corniculate, and cuneiform).
The extrinsic muscles of the larynx move the larynx as a whole. The infrahyoid muscles (omohyoid, sternohyoid, and sternothyroid) are depressors of the hyoid bone and the larynx, whereas the suprahyoid muscles (stylohyoid, digastric, mylohyoid and geniohyoid) and the stylopharyngeus are elevators of the hyoid bone and larynx.
The intrinsic muscles of the larynx are concerned with the movements of the laryngeal parts, making alterations in the length and tension of the vocal folds and in the size and shape of the rima glottidis in voice production. All intrinsic muscles of the larynx are supplied by the recurrent laryngeal nerve (RLN), a branch of the vagus nerve (CN X) except the cricothyroid muscle, which is supplied by the external laryngeal nerve.
The adductors of the vocal folds include the lateral cricoarytenoid muscles which arise from the lateral portions of the cricoid cartilage and insert into the muscular processes or the arytenoid cartilages. These muscles pull the muscular processes anteriorly, rotating the arytenoid cartilages so that their vocal processes swing medially. These movements adduct the vocal folds and close the rima glottidis.
The principle abductors of the vocal folds are the posterior cricoarytenoid muscles. These muscles arise on each side from the posterior surface of the lamina of the cricoid cartilage and pass laterally and superiorly to insert into the muscular processes of the arytenoid cartilages. They rotate the arytenoid cartilages, thereby deviating them laterally and widening the rima glottidis.
The main tensors of the vocal folds are the triangular cricothyroid muscles. These are located on the external surface of the larynx between the cricoid and thyroid cartilages. The muscle on each side arises from the anterolateral part of the cricoid cartilage and inserts into the inferior margin and anterior aspect of the inferior horn of the thyroid cartilage. These muscles tilt the thyroid cartilage anteriorly on the cricoid cartilage, increasing the distance between the thyroid and arytenoid cartilages. As a result, the vocal ligaments are elongated and tightened and the pitch of the voice is raised.
The principle relaxers of the vocal folds are the broad thyroarytenoid muscles. They arise from the posterior surface of the thyroid cartilage near the median plane and insert into the anterolateral surfaces of the arytenoid cartilages. One band of its inferior deep fibers, called the vocalis muscle, arises from the vocal ligament and passes to the vocal process of the arytenoid cartilages anteriorly. The thyroarytenoid muscles pull the arytenoid cartilages anteriorly, thereby slackening the vocal ligaments. The vocalis muscles produce minute adjustments of the vocal ligaments (e.g., as occurs during whispering). They also relax parts of the vocal folds during phonation and singing.
The laryngeal nerves are derived from the vagus nerve (CN X) through the superior laryngeal nerve and the RLN. All intrinsic muscles, except cricothyroid, are innervated by the RLN with fibers from the accessory nerve (CN XI). The external laryngeal nerve supplies the cricothyroid muscle. The supraglottic portion of the laryngeal mucosa is supplied by the internal laryngeal nerve, a branch of the superior laryngeal nerve. The infraglottic portion of the laryngeal mucosa is supplied by the RLN.
Dystonia is a movement disorder that can affect a single muscle group or the entire body. Dystonia is typically characterized by sustained muscular contraction that is forceful and inappropriate. Although dystonia has been linked to malfunction in certain areas of the brain, such as the basal ganglia, the precise cause of the disease remains unknown. Dystonias are generally classified into two groups, generalized and focal. Generalized (non-focal) dystonia involves a large number of muscle groups. Focal dystonia involves a single muscle group. The most common types of focal dystonia are blepharospasm, torticollis, writer's cramp, and laryngeal. Laryngeal dystonia, also called spasmodic dysphonia, is a focal, primary dystonia, affecting the muscles of the larynx.
Spasmodic dysphonia is an extremely disabling form of dystonia that is often misdiagnosed. Patients with spasmodic dysphonia have severely diminished vocal capacity. The voice can range from strangled and pressed to breathy and barely perceptible.